build: Fix 32-bit TLS detection with 64-bit-default gas on Solaris/x86

build: Fix 32-bit TLS detection with 64-bit-default gas on Solaris/x86

I've recently tested i386-pc-solaris2.11 bootstrap on Solaris 11/x86
with only the bundled tools (using /usr/gnu/bin/as from binutils 2.30 in
this case).  It failed compiling libgo/runtime/proc.c, creating invalid
assembly:

proc.s: Assembler messages:
proc.s:2092: Error: junk at end of line, first unrecognized character is `*'

        .globl  __emutls_v.*runtime.g

and several more errors.  This is completely unexpected since Solaris
does support TLS.  It turned out that 32-bit TLS detection in
gcc/configure had failed:

configure:25145: checking assembler for thread-local storage support
configure:25158: /usr/gnu/bin/as   --fatal-warnings -o conftest.o conftest.s >&5
conftest.s: Assembler messages:
conftest.s:6: Error: relocated field and relocation type differ in signedness
conftest.s:7: Error: @TLSLDM reloc is not supported with 64-bit output format
conftest.s:7: Error: junk `@tlsldm' after expression

which isn't unexpected given that the bundled gas has been configured
for x86_64-pc-solaris2.11, i.e. 64-bit-default.

This is easily fixed by explicitly passing --32 for the 32-bit case,
matching what is done for the 64-bit test.

Tested on i386-pc-solaris2.11 with 32-bit-default and 64-bit-default gas
as well as with /usr/bin/as, always correctly detecting TLS support.

	* configure.ac <i[34567]86-*-*>: Add --32 to tls_as_opt on Solaris.
	* configure: Regenerate.